There's an extension to GHC: 'Postfix Operators' . https://downloads.haskell.org/~ghc/8.6.1/docs/html/users_guide/glasgow_exts.... Turns out Hugs can do that too (in Hugs mode).
(!) :: Num a => a -> a -- note defined as monadic
(!) 0 = 1 -- lhs of equation must use prefix form
(!) n = n * ((n - 1) !) -- but rhs can use postfix
So (3 !) returns 6, etc. The parens are needed so that it's parsed as a section. I also took a leaf out of Oleg's book and built a variadic postfix operator http://okmij.org/ftp/Haskell/polyvariadic.html That is, (3 ~- 7) is parsed as 'subtract 3 from 7'. (3 ~-) is parsed as postfix negate 3 aka 'subtract 3 from zero'. It kinda worked, but the terms need a lot of explicit signatures and/or tricky type casting. AntC